#define _CRT_SECURE_NO_WARNINGS 1
#include<iostream>
using namespace std;
const int N = 5e4 + 10;
typedef long long ll;
ll l, m, n;
ll a[N];
ll calac(ll mid)
{
    ll ret = 0;
    for (int i = 0; i <= n; i++)
    {
        int j = i + 1;
        while (j <= n && a[j] - a[i] < mid)
        {
            j++;
        }
        ret += j - i - 1;
        i = j - 1;
    }
    return ret;
}
int main()
{
    cin >> l >> n >> m;
    for (int i = 1; i <= n; i++)
    {
        cin >> a[i];
    }
    a[n + 1] = l;
    n++;
    ll left = 1;
    ll right = l;
    while (left < right)
    {
        ll mid = (left + right + 1) / 2;
        if (calac(mid) > m)
        {
            right = mid - 1;
        }
        else
        {
            left = mid;
        }
    }
    cout << left << endl;
}